The recent need for passive power electronics with improved performance, high reliability, and reduced size and weight, has driven interest in ceramic film on metallic substrates in these applications. The ceramic films on metal foils, known as “film-on-foil” technology, in which ceramic films were deposited on base metal foils for embedding into a printed circuit board. The interest in “film-on-foil” technology exploits ceramic dielectrics, including the important properties, such as ferroelectric, piezoelectric, pyroelectric and electro-optic properties. These properties are utilized in manufacture of nonvolatile semiconductor memories, thin-film capacitors, pyroelectric infrared (IR) detectors, sensors, surface acoustic wave substrates, optical waveguides, and optical memories. Recently, there has been increased interest in applying “film-on-foil” to power electronics, such as capacitors with high capacitance required to work at high voltages. Applying the film-on-foil technology can substantially reduce the production cost and improve the volumetric and gravimetric efficiencies of the capacitors.
Important ferroelectric materials for thin-film applications are typically titanates and niobates with oxygen-octahedral structure types, such as the perovskite structure. Examples of such ferroelectric perovskites include lead titanate (PbTiO3), lead zirconate (PbZrO3), lead zirconate titanate [Pb(Zr,Ti)O3 or PZT], lead lanthanum titanate [(Pb,La)TiO3], lead lanthanum zirconate [(Pb,La)ZrO3], lead lanthanum zirconate titanate [(Pb,La)(Zr,Ti)O3 or PLZT], lead magnesium niobate [Pb(Mg1/3Nb2/3)O3], lead zinc niobate [Pb(Zn1/3Nb2/3)O3], strontium titanate (SrTiO3), barium titanate (BaTiO3), barium strontium titanate [(Ba,Sr)TiO3], barium titanate zirconate [Ba(Ti,Zr)O3], potassium niobate (KNbO3), potassium tantalate (KTaO3), and potassium tantalate niobate [K(Ta,Nb)O3]. Device applications of ferroelectric thin films require that bulk ferroelectric properties be achieved in thin films. The physical and chemical properties of the film (density, uniformity, stoichiometry, crystal structure, and microstructure) are extremely important. The utilization of ferroelectric thin films for electronic and optical applications has been hindered by the lack of production processes to form deposits of sufficient thickness.
Particular interest has shown that lead lanthanum zirconate titanate (Pb0.92La0.08Zr0.82Ti0.48O3, PLZT) films deposited on nickel or copper foils possessed excellent dielectric properties, which are promising for high power applications such as plug-in hybrid electric vehicles. In power electronics, capacitors with high capacitance are required to work at high voltages, typically in the range of 450 to 600 V. This requirement imposes an additional challenge to fabricate thicker films (>1 μm) that can withstand high voltage. However, in the fabrication process, the deposited films crack easily during heat treatment, due to the well-known critical thickness effect. Due to this effect per-layer thickness that can be achieved by conventional sol-gel methods is generally limited to about 0.2 μm, thus making these methods unattractive to industry if thicker films are needed.
It has been reported that the critical thickness of lead zirconate titanate (PZT) films can be substantially increased by introducing polyvinylpyrrolidone (PVP) into sol-gel solutions (H. Kozuka and S. Takenaka, J. Am. Ceram. Soc. 85 (11) (2002) 2696-2702) and barium titanate (H. Kozuka and M. Kajimura, J. Am. Ceram. Soc. 83 (5) (2000)1056-1062). The increased critical thickness of the PZT dielectric is attributed to the structural relaxation effect as PVP suppressed the condensation reaction because of the strong hydrogen bonds between the amide groups of PVP and the hydroxyl groups of the metalloxane polymers (H. Kozuka and M. Kajimura, J. Am. Ceram. Soc. 83 (5) (2000)1056-1062). However, thick films derived from PVP-containing solutions were generally found to be porous due to the thermal decomposition of PVP during heating (A. Yamano and H. Kozuka, J. Am. Ceram. Soc. 90 (12) (2007)). Pyrolysis temperature had been shown to have a significant impact on microstructure of the films derived from PVP-modified sol-gel process (Z. H. Du, J. Ma, and T. S. Zhang, J. Am. Ceram. Soc. 90 (3) (2007)).
The efficient removal of decomposition byproducts produced by processing aids during dielectric fabrication and the consolidation of the film raises significant processing issues.